As disclosed in JP 2004-164601A (US 2004/0080997A), a conventional electronic control unit (ECU) controls an internal combustion engine provided as a motive power source. The ECU has a microcomputer, which controls the engine and performs failure diagnosis.
The microcomputer checks whether a failure diagnosis execution condition (diagnosis condition) of each item of diagnosis among plural failure diagnosis items. The microcomputer performs failure diagnosis processing of each diagnosis item, for which the failure diagnosis execution condition is satisfied.
It is assumed that one trip of a vehicle covers a period from turning-on to turning-off of an ignition switch of the vehicle and the number of trips is incremented by one in the period from turning-on to turning-off of the ignition switch. It is further assumed that a diagnosis execution rate of the number of execution of diagnoses relative to the number of trips. The diagnosis execution rateBase monitor method defines that the diagnosis execution rate need be equal to or larger than a predetermined value. It is necessary to make more trips to perform the failure diagnosis processing so that a required diagnosis execution rate value may be attained.
It is proposed recently to drive a vehicle automatically. An ECU for such a vehicle is configured to drive the vehicle automatically so that the failure diagnosis execution condition is satisfied for attaining the required execution rate value. In this case, the ECU controls the engine differently from normal automatic driving thereby to satisfy the failure diagnosis execution condition. When the engine is controlled differently from the normal automatic driving, riding comfortableness of the vehicle is degraded.